Reactive oxygen species (ROS) and reactive nitrogen species (RNS) have been linked to many serious medical conditions. Metal ions, especially those of iron, contribute to the formation of these oxidizing species in biological systems, but the details of these reactions are not well understood, and several conflicting proposals have been put forward. In this proposal, carefully calibrated, advanced quantum chemical calculations will be used to assess several hypotheses concerning the role of metal ions in the formation of oxidizing species. The viability of several proposed species, including several with iron in the form of feryl (Fe(IV)) will be studied. The influence of solvent and chemical environment (i.e. chelation) on the energetics and pathways will be investigated.